Near-field optical microscopy (SNOM or NSOM, for the term “Scanning Near-field Optical Microscopy”) is a scanning microscopy technique which enables the limit imposed by diffraction on the resolution of conventional optical microscopy to be overcome. The principle on which this technique is based consists in illuminating a sample and in scanning its surface with a probe of nanometric size (generally a sharp point of a tip). The probe is usable for working in the optical near field, in collection and/or illumination mode, and thus allows an image of the sample to be obtained whose resolution is limited by its dimensions rather than by diffraction.
The probe can be a simple scattering tip, such as a tip of an atomic force or tunneling microscope, whose function is solely to convert the evanescent waves, present near to the sample, into propagating waves which can be detected in far-field mode by a conventional optical system. This is referred to as near-field optical microscopy “without aperture”.
As a variant, the probe can have an aperture of nanometric size: this is near-field optical microscopy “with aperture”. This nano-aperture can be used to generate evanescent waves that the sample can convert into propagating waves which will subsequently be detected in far-field mode (“illumination mode”), in order to collect evanescent waves generated by the sample illuminated by an external and non-local light source (“collection mode”), or else in order to generate and to collect evanescent waves at the same time (“illumination-collection mode”). The probe with a nano-aperture can be composed of an optical fiber having one drawn out and metalized end.
The conventional techniques of near-field optical microscopy—with or without aperture—do not allow a spatial resolution better than around 30 nm to be obtained. Since the spatial resolution is directly linked to the dimensions of the probe, apertures and/or tips with dimensions smaller than the desired resolution must be used. Although the use of apertures or tips with dimensions less than around 30 nm is technically achievable, in practice, probes of this type would be unusable because they would generate a signal of insufficient intensity with a low signal-to-noise ratio.
The concept of a near-field optical microscopy using an active probe, introduced in the 1990s, aims to provide an improvement in the spatial resolution. Its principle consists in using a secondary light source of nanometric dimensions, emitting directly in the near field of the object to be observed. As a source, a fluorescent nano-object may be used, for example a microcrystal containing a single fluorescent molecule, attached to the end of a scattering tip (J. Michaelis et al. “Optical microscopy using a single-molecule light source”, Nature, Vol. 405, 18th May 2000, pp. 325-328). In reality, the implementation of this concept proved to be very complex and did not allow resolutions to be obtained that were better than the more conventional techniques using “passive” probes. This is due mainly to the fact that it is difficult to attach and to maintain a nano-light source in a stable manner on a probe, and to accurately position it.
I. Berline et al. have proposed an approach allowing this difficulty to be avoided. As illustrated in FIG. 1, these authors use a metal tip PM immersed in a droplet of liquid L posed on the surface of the sample E to be observed. The liquid is a solution containing 4-di-butyl-amino-4′-nitroazobenzene (DBANA), a molecule with an elongated shape, exhibiting a permanent electric dipole and a high hyper-polarizability β (dielectric susceptibility of the second order at the molecular scale). A potential difference ΔV is applied between the metal tip and the sample; thus an intense static electric field is developed mainly at the apex of the tip and induces a localized alignment and orientation of the molecules. Simultaneously, the assembly formed by the tip and the liquid droplet is illuminated by a pulsed laser beam FL in the near infrared (wavelength λ1: 780 nm; duration: 100 fs). In a small volume VS immediately underneath the tip, whose lateral dimensions are of the order of the radius of curvature of the apex of the latter, the molecules of DBANA are oriented and aligned and emit coherent second harmonic radiation SH at a wavelength λ2=λ1/2=390 nm. Outside of this volume, the random orientation of the molecules of DBANA only allows the generation of very weak incoherent second harmonic radiation. Thus, the volume VS may be considered as a nano-source for light, emitting radiation at a wavelength different from that of the illuminating laser beam. In contrast to the nano-sources of light used in the previously known active SNOM probes, the volume VS is necessarily positioned exactly in correspondence with the apex of the tip PM; furthermore, no complex attachment operation is required. Moreover, a relatively intense light emission can be obtained starting from a small number of molecules—and hence from a very small volume VS—thanks to the coherent nature of the process of second harmonic generation (a signal that is quadratic with the number of oriented molecules), which is not the case for the nano-sources based on molecular fluorescence.
However, the use of a liquid droplet may be incompatible with certain samples. Furthermore, the refraction within the droplet and the absorption of the light by the liquid complicate both the illumination of the sample and the collection of the second harmonic signal. In order to get round this difficulty, it has been proposed to perform the illumination and the collection via the back face of the sample, for example by means of a transparent prism PR or of a microscope objective lens having a high aperture number. Such a configuration may not always be envisioned, especially where the sample is opaque.